1. The Field of the Invention
The present invention relates to a device that is used to test the bond strength of an adherend, which is formed from a hardenable material, and is bonded to a test piece. More particularly, the present invention is related to testing bond strength between an adherend and a substrate by shearing the adherend from the substrate.
2. The Prior State of the Art
The goal of forming a bond between restorative dental materials and dentin, enamel, or other dental substrates is to withstand the significant shear forces created in the oral environment and to reinforce the remaining tooth structure. For this reason, high bond strengths are desirable. Measuring the strength of a bond between restorative dental materials and dental substrates requires two steps. First, the bond between the dental materials must be formed and second, the restorative dental material must be sheared or pulled until failure occurs and then the peak force per unit area is determined.
The formation of a bond has several steps. First, the type of dental substrate, adhesive and adherend to be tested are selected. If the dental substrate is irregularly shaped, such as a tooth, then the tooth is mounted in resin to form a bonding substrate test piece. The test piece is cut or polished to create a smooth, flat top surface with a portion of the tooth exposed, preferably at the same level as the resin. The exposed portion of the tooth or other dental substrate is referred to herein as a test sample. Next, the top surface of the test sample is etched and typically rinsed to remove any contaminates. The test sample, or at least a portion thereof which will be the bond site, is then coated with a primer and/or adhesive, which will either be light cured or be allowed to chemical cure. A secondary restorative dental material (adherend) is then placed on the bond test site and is also light cured or chemical cured. The curing process hardens both the adhesives and the adherend. Once this process is completed, a bond has been formed between the restorative dental materials and the test sample or dental substrate such that a bond assembly now exists. The process of creating the bond assembly in the prior art presents several problems, which prevent the strength of the bond from being accurately measured.
The first problem associated with the bond assembly is related to the shape of the adherend. A cylindrical shaped adherend is the most conducive for obtaining an accurate measurement of the bond strength. Any other geometric shape, as well as deviations in the cylindrical shape lead to less accurate measurements of bond strengths. A perfect cylindrical shape however, has proven difficult to form as illustrated by the prior art. If the adherend is not formed to have a uniform cross-sectional shape as taken along its length, then a shear device used to shear the adherend for bond strength testing may not be able to properly interface with the adherend.
One prior art method involves bonding a composite filled gelatin capsule onto a testing substrate. In this method, a slightly overfilled gelatin capsule is overturned and manually or mechanically held in place on the substrate. The resulting adherend has a number of problems. First, a manually or mechanically held gelatin capsule is to some extent compressed. This compression deforms the cylindrical shape of the adherend. The second problem is that the gelatin capsule must be held immobile during the hardening process, which is difficult to do manually. This factor further deforms the shape of the composite material. The third problem is that the gelatin capsule must be slightly overfilled to ensure the proper adaptation of the composite to the substrate. When the composite filled capsule is placed onto the substrate the excess composite must be removed before curing takes place. This process creates difficulty in keeping the capsule stationary before curing which leads to further deformation. The combination of these deforming factors produces a composite material that is not perfectly cylindrically shaped and will not fit a shear device perfectly, which results in inaccurate bond strength measurements. A further difficulty is ensuring that the gelatin capsules are held perpendicularly to the dentin. If the adherend is not perpendicular to the substrate then the test loads will not be able to be applied properly.
Another attempt to eliminate these problems is the use of a small stainless steel nozzle, which is attachable to a guide fixture. The nozzle has small windows through which composite material can be added and through which the composite material is light cured. This method eliminates deformities in the shape of the composite material due to compression. However, it still has problems because the small windows cause difficulty in composite placement and curing. The windows limit the amount of light that can enter the nozzle to cure the composite material, which leads to inaccurate measurements of the bond strength because the composite material may not be completely cured. Removing the nozzle, after the curing process is difficult because the cured composite often extends into the windows, thus binding the nozzle in place. The difficulty in removing the nozzle creates stress on the newly formed bond and can weaken or fracture the bond. Note, the nozzle is held perpendicular to the substrate by means of a guide fixture, which also limits the user in choosing a suitable test site. Additionally, the stainless steel mold will not allow use of some restorative materials such as glass ionomers, copolymers, luting cements, and amalgam, thus limiting the ability to gather information related to such dental restorative materials. An example of such stainless steel instruments is the system sold under the name Bencor Multi-T. Information regarding this system is provided by a distributor, Danville Engineering at "www.danvilleengineering.com" which is linked to "www.edoc.co.za/dentalnet/research/microgrip/bencor.shtml" to provide more detailed information.
The next problem in the prior art is that the bond is not limited to the area between the test site and the adherend. When adhesives are applied to the substrate, it is typically applied to the entire surface of the substrate. This excess adhesive is not removed before the curing process occurs and results in a resin snowshoe or shelf, which encompasses more surface area than the test site. This resin snowshoe can distribute test loads out over the surface of the substrate; similar to the way a snowshoe spreads out the load of a human over a broader area. This resin snowshoe prevents the true strength of the bond from being measured. In essence the resin snowshoe bonds the adherend to more than just the test site. For this reason, the measurement of the bond strength is not accurate. The use of split molds, straws, or tubing such as TYGON.TM. tubing to create an adherend without creating a "resin snowshoe" requires the application of the primer and adhesive through the openings of these devices. When adhesives are applied in such a manner, capillary action occurs and some of the adhesive is drawn up the interior walls. Since this negatively influences the accuracy of test data, the result is inaccurate measurement of material properties.
Once the bond is formed between the substrate and the adherend, the strength of the bond can be tested. Testing the bond strength means measuring the force per unit area required to shear the adherend from the substrate. In addition to the factors related to the formation of the bond that effect test results, the actual testing of the bond could introduce inaccuracies. The prior art demonstrates additional problems that can influence the measurement of the bond strength.
The frictional force between the shear device and the test piece must be taken into account in order to obtain accurate bond strength measurement. The frictional force is typically greater when the shear device has a large amount of surface area in contact with the surface of the test piece or is held in place with guide fixtures. The shear device must load the bonded specimen until the adhesive fails without fracturing the adherend. If the adherend fractures first, then the adhesive is not the reason for failure and an accurate bond strength measurement cannot be obtained. The problem with fracturing the adherend rather than shearing the adhesive is more prominent when the adherend is not perpendicular to the substrate. If the shear device is too thin, then the adherend is, once again, more likely to be fractured and the resulting bond strength measurement is resultantly inaccurate. The shear force must be applied as close to the bond interface as possible or at the base of the adherend. If the shear force is not applied at the base of the adherend, a lever arm will be created and the force required to shear the bond will be measured inaccurately.
There are other methods of creating bond assemblies which maintain the controls necessary to obtain fairly accurate measurements but they are very cumbersome to use which limits productivity. These prior art systems do not offer the user ease of use, freedom of material choice, and choice of bond location while still maintaining the accuracy needed.
Researchers are employing many different methods of testing shear bond strength. Many of the methods involve complex fixturing which usually introduce more errors than benefits. An example of a method that involves complex fixturing is the method developed by Larry Watanabe, which is identified as ISO TR 11405.
There is a need in the industry for a method for both creating a bond assembly and testing the bond between adherend and the substrate such that the measurements of the bond strength actually represent the bond strengths.